“She has fallen! Babylon the Great has fallen, she who made all the nations drink of the wine of the passion of her sexual immorality!”
You're familiar with the goddess Dea Roma, right? You're aware that she was a female personification of Rome? You're aware that the Jews of the time period hated Rome and called it a second Babylon? And that they especially hated Dea Roma and called her the mother of prostitutes?
How would the target audience have interpreted the label, "Babylon the Great, the mother of harlots"? Would they have perhaps interpreted it within the social and political context in which it was written?